Use qemu/KVM virtualization instead of UML

Guest and network configuration is setup using the libvirt
virtualization API. The [start|stop]_testing scripts have been updated
accordingly.

qemu/KVM does not currently support a hostfs, so the shared build tree
mount has been dropped for now.

testing/config/kvm/vnet2.xml

@@ -0,0 +1,11 @@
+<network>
+  <name>vnet2</name>
+  <uuid>b5147a7d-e184-5c9e-3838-4621796ba95c</uuid>
+  <forward dev='lo' mode='route'>
+    <interface dev='lo'/>
+  </forward>
+  <bridge name='virbr2' stp='on' delay='0' />
+  <mac address='52:54:00:05:F3:34'/>
+  <ip address='10.1.0.254' netmask='255.255.0.0'>
+  </ip>
+</network>